Occurrence Rates of Electromagnetic Ion Cyclotron (EMIC) Waves With Rising Tones in the Van Allen Probes Data Set

In Fourier time-frequency power spectrograms of satellite magnetic field data, electromagnetic ion cyclotron (EMIC) waves may feature discrete, rising tone structures that rapidly increase in frequency. Using data from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer, we conducted a statistical study of EMIC waves from September 2012 through June 2016. We compared the occurrence rates and spatial distributions for all EMIC waves with those for rising tone EMIC waves as a function of magnetic local time (MLT) and L shell, as well as a function of R _XY and Z in solar-magnetic (SM) coordinates. Overall, EMIC waves occurred during 2.4% of the time period considered, but rising tone EMIC waves were only found during 0.2% of the time period considered. About 7%-8% of the minutes of orbital coverage with H+ or He+ band EMIC waves had rising tones. The regions of peak occurrence rates for H+ and He+ band waves, as well as waves with rising tones, were found in the noon and dusk sectors for 4 < L < 6. The preferred regions for H+ waves as a function of R _XY and Z SM suggest an association with magnetospheric compressions near noon and interactions between plumes and the ring current near dusk. Peak occurrence rates for O+ band waves were found between 2 < L < 4 at all MLT, and over a wide range of L shells near dusk. No rising tones were found in the O+ band.

Prompt Response of the Dayside Magnetosphere to Discrete Structures Within the Sheath Region of a Coronal Mass Ejection

A sequence of discrete solar wind structures within the sheath region of an interplanetary coronal mass ejection on November 6, 2015, caused a series of compressions and releases of the dayside magnetosphere. Each compression resulted in a brief adiabatic enhancement of ions (electrons) driving bursts of electromagnetic ion cyclotron (EMIC; whistler mode chorus) wave growth across the dayside magnetosphere. Fine-structured rising tones were observed in the EMIC wave bursts, resulting in nonlinear scattering of relativistic electrons in the outer radiation belt. Multipoint observations allow us to study the spatial structure and evolution of these sheath structures as they propagate Earthward from L1 as well as the spatio-temporal characteristics of the magnetospheric response. This event highlights the importance of fine-scale solar wind structure, in particular within complex sheath regions, in driving dayside phenomena within the inner magnetosphere.